The present invention generally relates to an apparatus for use in thermomechanical analysis and, in particular, relates to an apparatus adapted to perform both static and dynamic thermomechanical measurements without changing the loading means.
Apparatus of this kind is employed to investigate the mechanical behavior of, inter alia, plastics or plastic film coatings as a function of load and temperature. Conventionally, measurements are performed over a temperature range extending from a temperature far below the glass transition temperature to a temperature near the distortion temperature. Static loading techniques are generally employed, depending on the size of the load, for measuring the expansion of the sample or the depth of penetration of the probe into the sample. Dynamic loading techniques are usually used to determine the elastic, the plastic or the permanent deformation of the sample. Knowledge of such properties is important for the investigation of the mechanical properties of most materials, for example lacquers.
One conventional apparatus is described by K. H. W. Reichert and G. Donnebrink as published under the title "Eine modifizierte Form der thermomechanischen Analysen zur Messung der temperaturabhangigen Eigenschaften von Anstrichfilmen" in the Journal "farbe+lack", vol. 86, issue no. 7, page 591, in 1980. In the apparatus described therein, the load member forms a piston-like probe one end of which is supported by the sample in a sample tube and the other end of which carries a weight tray, the sample becoming statically loaded by weights put onto the latter. The sample tube is provided with a gas introduction means and with cooling means while the sample is provided with a thermocouple. The piston-like probe is guided by guiding means and includes a magnet member forming a solenoid plunger which coacts with an axially displaceable measuring coil forming the position signalling means. The measuring coil encircles the solenoid plunger to form a displacement transducer the output signals of which indicate the displacement of the solenoid plunger from a zero position. These signals are recorded by any recorder means. A compensation spring is used to compensate the piston-like probe for its own weight. In the same apparatus the sample may also be loaded dynamically. However, to accomplish the dynamic loading, the armature of an electromagnet is placed on the weight tray. The armature is adapted to periodically oscillate by an electromagnet which is excited according to selectable periodic functions of time. Thus, the oscillations effectively provide alternating load and relief pulses to the sample. The oscillator characteristic of the piston-like probe is affected by the mechanical properties of the sample and by the temperature and are also recorded by the recorder.
In adapting the apparatus from measuring static loads to dynamic loads, a change in the loading means is required. The measuring coil must be readjusted after each exchange of the loading means because of the changes in the weight of the load member. Further, care must be exercised for the armature and the electromagnet to assume the same relative initial position as far as possible since otherwise the loads eventually may not be comparable. In addition, to avoid erroneous results operation must remain within the effective range of the compensating spring and that this range can not be exceeded during the application of the load to the sample.